Not trying to be on the kids side (I don’t have kids and I’m happy I didn’t have to deal with myself as a kid), but if you’re saying they can just unistall the app when they are at their mom’s, then why would they reinstall the app when they are with you.
Speaking from my own experience: I wouldn’t have done that ever.
If you want kids off their devices: provide alternative entertainment.
This topic may give you some ideas though: Stop internet to certain devices